1. Field of the Invention
The invention relates to a discharge lamp with a unidirectional base and a unidirectional, sealed end which has a front cover and a reflector. A lamp of this type is used for light filament illumination using optical fibers, for spot lighting, such as shop lighting or the like, and for a light source for purposes of projection, which is installed in a projector, such as an OHP, liquid crystal projector and the like.
2. Background of the Disclosure
Ordinarily a halogen lamp with unidirectional base and unidirectional, sealed end together with a reflector is used for light filament lighting using optical fibers, for spot lighting in shop lighting or the like, or for a light source for purposes of projection of an OHP, liquid crystal projector or the like. In the case in which a halogen lamp is used as a light source, the following disadvantages however arise:
1) The lighting intensity which is obtained with regard to starting power is low. To obtain sufficient lighting intensity on a projection surface, it is necessary to provide especially strong power for the lamp. PA1 2) The fight emitted from the lamp contains a large amount of infrared radiation. In the case in which the lamp is installed in devices of different types, it is therefore necessary to use, at the same time, an infrared absorption filter, an infrared reflection filter and the like in order to reduce the temperature on an irradiated surface or within a device. PA1 3) To obtain good color reproduction, the color temperature of the lamp must be set relatively high. In this case, however, due to burn-out of a filament, the service life of the lamp is shortened. Burn-out of the filament takes place, for example, after 35 to 50 hours when the color temperature of the lamp is set to roughly 3200.degree. K. PA1 1) The thickness of a bulb which forms the emission part of the metal halide lamp, T (mm), the distance between the electrodes thereof, L (mm), and the lighting voltage of the lamp, V (volt), bear the following relationship to each other: EQU 10&lt;V/(L*T)&lt;25 PA1 2) The outside diameter of a front side of the bulb which forms the emission part of the metal halide lamp, D.sub.1 (mm), the outside diameter of a side of the above-described bulb, D.sub.2 (mm), the length of the above-described bulb, D.sub.3 (mm), and the lighting power of the lamp, W (watt), bear the following relationship to each other: EQU 0.07&lt;W/(D.sub.1 *D.sub.2 *D.sub.3)&lt;0.20 PA1 3) The volume of an area which is surrounded by the front cover and the reflector, Q.sub.1 (cm.sup.3), and the volume of the bulb which forms the emission part of the above-described metal halide lamp, Q.sub.2 (cm.sub.3), bear the following relationship to each other: EQU Q.sub.1 /Q.sub.2 &lt;15 PA1 4) The thickness of the bulb which forms the emission part of the metal halide lamp, T (mm), the distance between the electrodes thereof, L (mm), the lighting voltage of the lamp V (volt), the outside diameter of a front side of the above described bulb, D.sub.1 (mm), the outside diameter of a side of the above-described bulb, D.sub.2 (mm), the length of the above-described bulb D.sub.3 (mm), and the lighting power of the lamp W (watt), bear the following relationship to each other: EQU 10&lt;V/(L*T)&lt;25 EQU 0.07&lt;W/(D.sub.1 *D.sub.2 *D.sub.3)&lt;0.20 PA1 5) T.sub.2 /T.sub.1 is greater than or equal to 1.6, where T.sub.1 (mm) is the thickness of the bulb which forms the emission part of the above-described lamp, and T.sub.2 (mm) is the thickness of the glass of the front cover, in the case in which the operating pressure of the lamp during illumination is 3.times.10.sup.6 Pa, the inside volume of the lamp is 1 cm.sup.3 and the distance between the light source and the front cover glass is 20 mm. PA1 1) First, the inventors found that by defining the thickness of a bulb which forms the emission part of the lamp, the distance between the electrodes and the lighting voltage of the lamp, a still smaller probability of breakage of the lamp than the conventional PPM level can be obtained. PA1 2) Second, the inventors found that by defining the size of the bulb which forms the emission part of the lamp and the lighting voltage of the lamp, an even more advantageous lamp characteristic, especially good color reproduction, can be achieved. PA1 3) Third, the inventors found that also by defining the ratio between the volume of the area which is surrounded by the front cover and the reflector and the volume of the bulb which forms the emission part of the lamp, within an optimal numerical range illumination with advantageous lamp characteristics, especially with good color temperature and good color reproduction, can be effected. PA1 4) Fourth, the inventors have found that in the case of a relatively small lamp shape, by defining the ratio between the thickness of the bulb which forms the emission part of the lamp and the thickness of the front cover, safety can be adequately guaranteed, even if the lamp breaks during illumination.
Based on the above-described circumstances, a metal halide lamp which is installed in a reflector is used instead of a halogen lamp. A metal halide lamp of this type is more advantageous than a halogen lamp with respect to high efficiency, good color reproduction and high power. However, for purposes of stabilization of the outside peripheral temperature of the lamp during illumination or for similar purposes, it has a double tube arrangement in which there is one outside tube. In this case, the device is rather large as a whole if a metal halide lamp of the double tube type is installed in the reflector.
In addition, a metal halide lamp with a bilateral base and with bilateral sealed ends can be installed in a reflector without providing an outside tube. In this case, however, the lamp as a whole has a greater length than for a unidirectional base, and as a result a large reflector is needed, or the disadvantage arises that the tip of the lamp projects out of the front opening of the reflector if a smaller reflector is used.
On the other hand, there are cases in which a front cover, such as transparent glass or the like, is provided in the front opening of the reflector. This front cover can prevent fouling of the lamp surface or reflecting surface of the reflector as a result of adhesion of dirt. The front cover can, furthermore, prevent shifting of the lamp position by contact with other parts, even when an integrated reflector/lamp arrangement is installed in a device, such as a projector or the like. In addition, the front cover can minimize damage, even if the lamp breaks, although the possibility of breaking of a metal halide lamp during illumination is generally on the order of 1 to 1 million, subsequently called the "PPM level", and is extremely low. It is, therefore, desirable to provide a front cover for the one-part arrangement of the metal halide lamp and reflector which is arranged such that it surrounds the metal halide lamp.